1. Field of the Invention
The present invention relates to a soft magnetism thin film inductor and a magnetic multi-element alloy film.
2. Description of Related Art
With rapid progress in information technology (IT) industry, a soft magnetism thin film is extensively applied to a RF-band, particularly in a range between 800 MHz and 6 GHz. For example, the soft magnetism thin film may be applied to an integrated passive device, an electromagnetic noise protection measure, a sensor, and so on. Specifically, in terms of wireless communication applications, an operating frequency of a wireless local area network (WLAN) system has attained to a GHz-band, so as to deal with mass data transmission, such as Bluetooth and IEEE802.11b in a 2.45 GHz frequency band, IEEE802.11a in a 5.8 GHz frequency band, and so forth.
On the other hand, in order to enhance portability of mobile communication devices and to integrate multiple functions thereof, miniaturization of mobile phone components is one of the focuses in relevant research and development. Thus, the size of indispensable passive devices including thin film inductors and multilayer capacitors in the electronic devices is reduced little by little. Here, the fabrication of the thin film inductors has called for significant attention.
Nowadays, a high-frequency inductor is mainly fabricated with use of ferrite powder (ceramic ferromagnetic materials), so as to avoid generation of eddy currents when the inductor is operated in high frequency. In the manufacturing process, the ferrite powder is first sintered at a high temperature and then bonded to a circuit board through performing a surface mounting technology (SMT). The most advantageous feature of the ceramic ferromagnetic material lies in its high resistivity, whereas other characteristics possessed by the ceramic ferromagnetic material are not beneficial for high-frequency communication applications. For example, saturation magnetization of the ceramic ferromagnetic material is lower than that of a metallic ferromagnetic material, and thus a restriction of Snoek's limit may be imposed on the ceramic ferromagnetic material in a high-frequency operation. A magnetic permeability value of the ceramic ferromagnetic material is less than 5 GHz. Moreover, since a maximum temperature at which a Si integrated circuit is manufactured is 500° C., integration of ferrite passive devices to a single chip is also demanding.
On the other hand, the inductor may also be fabricated by utilizing a conventional permalloy thin film. In spite of great saturation magnetization, the relatively low resistivity of the permalloy thin film results in significant loss of the eddy currents in the high-frequency operation, bringing about nonoccurrence of magnetic effects. To achieve favorable magnetic permeability in the high-frequency operation, new soft magnetism alloys including FeTaN, FeBSi, CoNbZr and FeAlO have been launched recently. However, several issues associated with the soft magnetism alloys are to be resolved. For example, since a FeTaN thin film and a CoNbZr thin film have excessively low magnetic anisotropy fields, as the frequencies thereof are less then 100 MHz, the values of the magnetic permeability are rapidly decreased. Further, a FeBSi thin film may still have iron loss in the high-frequency operation due to its relatively low resistance value (approximately 150 μΩ-cm) reducing induction efficiency. With respect to researches on improvement of the high-frequency characteristics possessed by the thin film inductor, variations in a magnetic flux generated when currents pass through conductors are amplified by extensively adopting magnetic materials. Thereby, inductance and a quality factor (Q factor) can be improved. For example, in U.S. Pat. No. 3,413,716, it is proposed to form a ferrite layer on a conductive layer of the thin film inductor through a physical deposition performed on the thin films, such that the Q factor of the thin film inductor can be enhanced. However, as the frequency exceeds 100 MHz, the magnetic permeability is expeditiously reduced, and therefore it is unlikely for the thin film inductor device to improve inductance and the Q factor by means of magnetic amplification in the high-frequency operation.
Besides, in the researches on adding the magnetic materials during the process of making the thin film inductor, the high-frequency characteristics of the thin film inductor may also be improved through a structural design thereof. For example, in U.S. Pat. No. 6,373,369 B2, a cylindrical magnetic material located at the center of a spiral conductor is disclosed, and the cylindrical magnetic material is not in contact with the spiral conductor for improving the high-frequency characteristics possessed by the thin film inductor. Nevertheless, the complicated shapes of the thin film inductor and the intricate manufacturing process thereof raise the costs of fabricating the magnetic material. On the other hand, in U.S. Pat. No. 6,822,548 B2, the magnetic material encasing coils of the thin film inductor is not arranged in sequence. Air gaps formed in the magnetic material divide the same into sections, so as to prevent loss of the eddy currents in the high-frequency operation. However, in the thin film inductor, the magnetic material does not completely cover the conductive layer, and thus an improvement of inductance per unit area is restricted. The requirement for complicated shapes also results in higher manufacturing costs of the magnetic material.
Furthermore, Japanese Patent No. 5,101,930 provides a highly saturated magnetic flux layer and a soft magnetism layer which are alternately stacked, e.g. a multi-layered film having stacked FeBN/FeN. The multi-layered structure is capable of efficiently enhancing saturation magnetization, while the resistance value is still insufficient. Accordingly, loss of the eddy currents leads to a rapid decrease in the Q factor, such that the multi-layered structure is unlikely to be applied in the high-frequency operation.